N Tier Architecture

Hi there,
I've developping an online shopping system, including web interfaces, a back end windows form to take care of orders, sales, and stock management. Also there is a web service to return stock on hand of a product. That's all.
I am thinking if I can fully implement the following architecture for this system. Here is the diagram
http://img131.exs.cx/img131/2540/f02aa01.gif
(source: http://msdn.microsoft.com/library/de...AppArchCh2.asp )

Followings are description of how I would map to the online shopping app:
1) just web pages and window form
2) processes such as: if a user not yet authenticated, order button should not be anabled, manipulate datagrid to display products catalogs.
3) Business workflow: keeps track of the work flow in the system (add items to shopping cart, login, credit card details, shipment arrangement)
4) Logics for both Web pages and Windows forms.
5) Talks to the check stock on hand service.
6)
7) ADO.net
8) Objects: Orders, items, users, ..
9) Security: role based security (not CAS).

I really have no idea about the blocks 3,6,9
+ Do you think number 3 is important to have it in our model?
+ Is there any way of implementing the idea of Service interfaces (number 6) in our model?
+ Number 9: I think the role based security specifies the role of the user (customer / admin) and therefore affects the behaviors of the other layers (as we can see, security spreads over the 3 layers), but it does not actually physically intact with the other layers, or have references (.dll) to those layers. Am I right?
+ Do you think any blocks in the model can be omitted?

Thanks
Patrick